Constant voltage power supply

ABSTRACT

A constant voltage power supply for supplying power to a load that switches between an active state and a standby state is disclosed. The constant voltage power supply includes first and second constant voltage circuits different in transient response and current consumption. The input of each of the first and second constant voltage circuits is connected to the input terminal of the constant voltage power supply, and the output of each of the first and second constant voltage circuits is connected to the output terminal of the constant voltage power supply. A switching signal generation circuit outputs a switching signal so as to cause the first operational amplifier to operate when the load is in the active state, and to cause the second operational amplifier to operate when the load is in the standby state.

TECHNICAL FIELD

The present invention relates generally to constant voltage powersupplies, and more particularly to a constant voltage power supplysupplying power to a load that switches between an active state and astandby state.

BACKGROUND ART

A constant voltage power supply that has a constant voltage circuit tosupply stable voltage is employed as a power supply for, for instance,cellular phones. The constant voltage power supply has a constantvoltage circuit that consumes a large amount of current (a high-speedconstant voltage circuit) in order to improve power supply rejectionratio (PSRR), or ripple rejection, and load transient response.Accordingly, when the constant voltage power supply is applied to anapparatus whose load has an active mode (active state) and a sleep mode(standby state), such as a cellular phone, the amount of unnecessarilyconsumed current is increased in the sleep mode, which does not requirehigh PSRR and load transient response. Therefore, consideration may begiven of a constant voltage power supply having a high-speed constantvoltage circuit and a constant voltage circuit that is inferior in PSRRand load transient response but reduces current consumption (a low-speedvoltage circuit), the constant voltage power supply having the functionof switching the constant voltage circuits based on the state of theload. In the low-speed constant voltage circuit, PSRR and load transientresponse are reduced because of reduced current consumption, but noproblem is caused when the load is in the sleep mode.

Japanese Laid-Open Patent Application No. 2001-117650 discloses aconstant voltage power supply having a high-speed constant voltagecircuit and a low-speed constant voltage circuit. FIG. 1 is a circuitdiagram illustrating this constant voltage power supply. A constantvoltage circuit 21 is provided to stably supply power from a powersupply 1 to a load 3 such as a cellular phone. The power supply 1 isconnected to an input terminal (Vbat) 23 provided to the constantvoltage circuit 21. The input terminal 23 is connected to an outputterminal (Vout) 27 through an output transistor (DRV) 25 composed of ap-channel MOS transistor. In the constant voltage circuit 21, ahigh-speed voltage stabilization part 29 a that consumes a large amountof current but has good PSRR and load transient response, and alow-speed voltage stabilization part 29 b that has inferior PSRR andload transient response but consumes less current are provided inparallel. A transistor size employed in the high-speed voltagestabilization part 29 a is greater in current supply capacity than thatemployed in the low-speed voltage stabilization part 29 b. In this case,the high-speed voltage stabilization part 29 a and the low-speed voltagestabilization part 29 b have the same circuit configuration, but aredifferent in response performance because of the difference in magnitudebetween currents supplied to respective operational amplifiers 33 a and33 b thereof. The high-speed voltage stabilization part 29 a is quickerin response than the low-speed voltage stabilization part 29 b.

The high-speed voltage stabilization part 29 a includes the operationalamplifier 33 a. The output terminal of the operational amplifier 33 a isconnected to the gate of the output transistor 25 through a switch part37 a provided to the constant voltage circuit 21. A reference voltage isapplied to the inverting input terminal of the operational amplifier 33a from a reference voltage part (Vref) 31 a. A divided voltage obtainedby dividing the output voltage of the output transistor 25 betweenvoltage-dividing resistors R1 and R2 is applied to the non-invertinginput terminal of the operational amplifier 33 a. Power to theoperational amplifier 33 a and the reference voltage part 31 a issupplied from the power supply 1. An n-channel MOS transistor serving asan interruption circuit 35 a that performs ON/OFF control of throughcurrent is provided between ground and each of the operational amplifier33 a, the reference voltage part 31 a, and the ground-side terminal ofthe resistor R2.

The low-speed voltage stabilization part 29 b, which has the sameconfiguration as the high-speed voltage stabilization part 29 a,includes a reference voltage part 31 b, the operational amplifier 33 b,an interruption circuit 35 b, and resistors R3 and R4 corresponding tothe reference voltage part 31 a, the operational amplifier 33 a, theinterruption circuit 35 a, and the resistors R1 and R2, respectively, ofthe high-speed voltage stabilization part 29 a. The output terminal ofthe operational amplifier 33 b is connected to the gate of the outputtransistor 25 through a switch part 37 b provided to the constantvoltage circuit 21. The operational amplifier 33 b consumes less currentthan the operational amplifier 33 a, so that the low-speed voltagestabilization part 29 b is inferior to the high-speed voltagestabilization part 29 a in PSRR and load transient response.

A switching logic circuit (SWITCHING LOGIC) 39 that outputs switchingsignals to the switch parts 37 a and 37 b is connected to the load 3.The switch parts 37 a and 37 b control connection and disconnectionbetween the output terminals of the respective operational amplifiers 33a and 33 b and the gate electrode of the output transistor 25. Whenhigh-level switching signals are input to the switch parts 37 a and 37b, the switch parts 37 a and 37 b connect the output terminals of therespective operational amplifiers 33 a and 33 b to the gate electrode ofthe output transistor 25. When low-level switching signals are input tothe switch parts 37 a and 37 b, the switch parts 37 a and 37 bdisconnect the output terminals of the respective operational amplifiers33 a and 33 b from the gate electrode of the output transistor 25. Theswitching logic circuit 39 is also connected to the interruptioncircuits 35 a and 35 b. The switching logic circuit 39 controls theoperations of the interruption circuits 35 a and 35 b in accordance withsignal inputs to the switch parts 37 a and 37 b, respectively. In thisconstant voltage power supply, the constant voltage circuit 21 indicatedby a broken line is formed on a single chip. The high-speed voltagestabilization part 29 a and the output transistor 25 form a firstconstant voltage circuit, and the low-speed voltage stabilization part29 b and the output transistor 25 form a second constant voltagecircuit.

Next, a description is given of the operation of the conventionalconstant voltage power supply. When the load 3 is in an active mode(active state), the switching logic circuit 39 outputs a high-levelswitching signal to the switch part 37 a and the interruption circuit 35a, and a low-level switching signal to the switch part 37 b and theinterruption circuit 35 b. As a result, the switch part 37 a and theinterruption circuit 35 a are connected so as to turn on the high-speedvoltage stabilization part 29 a, and the switch part 37 b and theinterruption circuit 35 b are disconnected so as to turn off thelow-speed voltage stabilization part 29 b (standby state). Consequently,the voltage applied to the gate electrode of the output transistor 25 iscontrolled by the high-speed voltage stabilization part 29 a. The amountof current consumed by the low-speed voltage stabilization part 29 b inits standby state is less than or equal to 1 μA.

When the load 3 is in a sleep mode (standby state), the switching logiccircuit 39 outputs a low-level switching signal to the switch part 37 aand the interruption circuit 35 a, and a high-level switching signal tothe switch part 37 b and the interruption circuit 35 b. As a result, theswitch part 37 a and the interruption circuit 35 a are disconnected soas to turn off the high-speed voltage stabilization part 29 a (standbystate), and the switch part 37 b and the interruption circuit 35 b areconnected so as to turn on the low-speed voltage stabilization part 29b. Consequently, the voltage applied to the gate electrode of the outputtransistor 25 is controlled by the low-speed voltage stabilization part29 b. The amount of current consumed by the high-speed voltagestabilization part 29 a in its standby state is less than or equal to 1μA.

When the operating modes are switched, the switching logic circuit 39generates a period during which the high-speed voltage stabilizationpart 29 a and the low-speed voltage stabilization part 29 b, bothcontrolling the operation of the output transistor 25, aresimultaneously turned on. When the load 3 is switched from the activemode to the sleep mode, the load 3 transmits a mode switching signal tothe switching logic circuit 39. As a result, the switching logic circuit39 turns on the low-speed voltage stabilization part 29 b, and after thepassage of a predetermined period of time thereafter, turns off thehigh-speed voltage stabilization part 29 a, thereby performing switchingto control by the low-speed voltage stabilization part 29 b.Consequently, the high-speed voltage stabilization part 29 a is notselected and enters the standby state.

When the load 3 is switched from the sleep mode to the active mode, theload 3 transmits a mode switching signal to the switching logic circuit39. As a result, the switching logic circuit 39 turns on the high-speedvoltage stabilization part 29 a, and after the passage of apredetermined period of time thereafter, turns off the low-speed voltagestabilization part 29 b, thereby performing switching to control by thehigh-speed voltage stabilization part 29 a. Consequently, the low-speedvoltage stabilization part 29 b is not selected and enters the standbystate. Thus, by generating the period of the “simultaneous ON state” atthe time of switching from the low-speed voltage stabilization part 29 bto the high-speed voltage stabilization part 29 a and from thehigh-speed voltage stabilization part 29 a to the low-speed voltagestabilization part 29 b, it is possible to prevent noise resulting froma great variation in the output Vout at the time of switching.

In some cases, however, a certain degree of load transient response andsupply voltage variation response (response to supply voltage variation)is required even in the sleep mode, although not as much as in theactive mode. The operational amplifier 33 b of the low-speed voltagestabilization part 29 b employed in the conventional technology reducescurrent consumption at the sacrifice of response speed. Further, thecurrent supply capacity of the output-stage buffer transistor of theoperational amplifier 33 b is also reduced. Controlling the outputtransistor 25, having such a large gate area as to be able to controllarge current, by such an operational amplifier results in extremelyslow response speed. Although the operation amplifier 33 b is of thelow-speed voltage stabilization part 29 b, its current consumptioncannot be reduced significantly if a certain degree of response speed isto be ensured.

Further, the two changeover switches (the switch parts 37 a and 37 b)are required to switch an output to be connected to the gate of theoutput transistor 25 between the outputs of the two operationalamplifiers 33 a and 33 b, thus resulting in a complicated circuit.Furthermore, when current has been continuously supplied to the load 3at the time of switching, the driver (output transistor 25) iscontrolled by the operation of the high-speed voltage stabilization part29 a having a large current supply capacity. Accordingly, a relativelyhigh level of noise may be generated during a certain period oftransition of the high-speed voltage stabilization part 29 a from an OFFstate to a stably operating state.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to providea constant voltage power supply in which the above-describeddisadvantages are eliminated.

A more specific object of the present invention is to provide a constantvoltage power supply that can be free of the complication of theconventional constant voltage power supply and can improve loadtransient response and supply voltage variation response in a standbymode without increasing current consumption.

The above objects of the present invention are achieved by a constantvoltage power supply for supplying power to a load that switches betweenan active state and a standby state, which includes a first constantvoltage circuit configured to apply a reference voltage to a first inputterminal of a first operational amplifier, apply a voltage obtained bydividing an output voltage to a second input terminal of the firstoperational amplifier, and control a first output transistor by anoutput of the first operational amplifier; a second constant voltagecircuit configured to apply a reference voltage to a first inputterminal of a second operational amplifier, apply a voltage obtained bydividing an output voltage to a second input terminal of the secondoperational amplifier, and control a second output transistor by anoutput of the second operational amplifier, the second constant voltagecircuit being configured to be inferior in transient response to andconsume less current than the first constant voltage circuit; and aswitching signal generation circuit configured to transmit a switchingsignal in accordance with the state of the load, wherein an input ofeach of the first and second constant voltage circuits is connected toan input terminal of the constant voltage power supply, and an output ofeach of the first and second constant voltage circuits is connected toan output terminal of the constant voltage power supply; and theswitching signal generation circuit outputs the switching signal tocause the first operational amplifier to operate when the load is in theactive state, and outputs the switching signal to cause the secondoperational amplifier to operate when the load is in the standby state.

According to one aspect of the present invention, a first constantvoltage circuit that consumes a large amount of current but hasexcellent ripple rejection and load transient response and a secondconstant voltage circuit that is inferior in ripple rejection and loadtransient response but consumes less current are connected in parallel.The first constant voltage circuit is caused to operate when a load isin an active state, and the second constant voltage circuit is caused tooperate when the load is in a standby state. As a result, it is possibleto improve current consumption by the power supply circuit when the loadis in the standby state. Further, the output transistor of the secondconstant voltage circuit is reduced in size. Accordingly, there is nosignificant decrease in response, which can be much better thanconventionally. Moreover, since the output transistor of the secondconstant voltage circuit is reduced in size, it is possible to preventan increase in IC chip area.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram illustrating a conventional constant voltagepower supply;

FIG. 2 is a circuit diagram illustrating a constant voltage power supplyaccording to an embodiment of the present invention; and

FIG. 3 is a timing chart for illustrating mode switching according tothe embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A description is given below, with reference to the accompanyingdrawings, of an embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a constant voltage power supplyaccording to the embodiment of the present invention. The constantvoltage power supply includes a first (high-speed) constant voltagecircuit 110 a and a second (low-speed) constant voltage circuit 110 b,each of which converts an input voltage (Vin) into a predeterminedvoltage and outputs the predetermined voltage. The inputs of the firstand second constant voltage circuits 110 a and 110 b are connected inparallel to an input terminal (Vin) 100, and the outputs of the firstand second voltage circuits 110 a and 110 b are connected in parallel toan output terminal (Vout) 130. A power supply such as a battery (notgraphically illustrated) is connected to the input terminal 100 of theconstant voltage power supply. Further, a load 150 that is an apparatussuch as a cellular phone is connected to the output terminal 130. Theload 150 has an active mode (an active state) and a sleep mode (astandby state).

The first constant voltage circuit 110 a includes a reference voltagepart 112 a generating a reference voltage (Vref1) (the reference voltagepart 112 a is also indicated as Vref1 in FIG. 2 for convenience ofgraphical representation), an operational amplifier (AMP1) 114 a, anoutput transistor (M1) 116 a, two resistors (R1 and R2) 118 a and 120 afor output voltage detection, and an n-channel MOS transistor (M2) 122a. The input terminal 100 is connected to the output terminal 130through the output transistor 116 a composed of a p-channel MOStransistor. The reference voltage part 112 a includes a Zener diode.Power to the operational amplifier 114 a and the reference voltage part112 a is supplied from the input terminal 100. The n-channel MOStransistor 122 a serving as an interruption circuit (a switchingcircuit) that performs ON/OFF control of through current is providedbetween ground and each of the operational amplifier 114 a, thereference voltage part 112 a, and the ground-side terminal of theresistor 120 a. The n-channel MOS transistor 122 a is turned ON to allowthe through current to flow, and is turned OFF to interrupt the throughcurrent. The reference voltage Vref1 is applied to the inverting input(−) of the operational amplifier 114 a. A divided voltage obtained bydividing the output voltage Vout between the detection resistors 118 aand 120 a is applied to the non-inverting input (+) of the operationalamplifier 114 a. The output of the operational amplifier 114 a isconnected to the gate of the output transistor 116 a.

The second constant voltage circuit 110 b includes a reference voltagepart 112 b generating a reference voltage (Vref2) (the reference voltagepart 112 b is also indicated as Vref2 in FIG. 2 for convenience ofgraphical representation), an operational amplifier (AMP2) 114 b, anoutput transistor (M4) 116 b, two resistors (R3 and R4) 118 b and 120 bfor output voltage detection, and an n-channel MOS transistor (M3) 122b. The input terminal 100 is connected to the output terminal 130through the output transistor 116 b composed of a p-channel MOStransistor.

A switching logic circuit (SWITCHING LOGIC) 140 (a switching signalgeneration circuit) outputs a first switching signal 140 a and a secondswitching signal 140 b to the first and second constant voltage circuits110 a and 110 b, respectively, in accordance with the state of the load150. The first switching signal 140 a is input to the gate of then-channel MOS transistor 122 a and the chip-enabling terminal (CE1) ofthe operational amplifier 114 a so as to control the operation of thefirst constant voltage circuit 110 a. The second switching signal 140 bis input to the gate of the n-channel MOS transistor 122 b and thechip-enabling terminal (CE2) of the operational amplifier 114 b so as tocontrol the operation of the second constant voltage circuit 110 b.

The first and second constant voltage circuits 110 a and 110 b have thesame configuration and operate in the same manner. The first and secondconstant voltage circuits 110 a and 110 b are connected in parallel. Thesecond constant voltage circuit 110 b is configured so as to be inferiorin transient response to but consume less current than the firstconstant voltage circuit 110 a. Therefore, the transistors forming thesecond constant voltage circuit 110 b have a smaller current supplycapacity than those employed in the first constant voltage circuit 110a. Accordingly, the second constant voltage circuit 110 b has lowerresponse speed than the first constant voltage circuit 110 a. The firstconstant voltage circuit 110 a consumes a large amount of current, buthas excellent PSRR or ripple rejection and load transient response. Thesecond constant voltage circuit 110 b is inferior in ripple rejectionand load transient response, but consumes less current.

The switching logic circuit 140 transmits the first and second switchingsignals 140 a and 140 b to the first and second constant voltagecircuits 110 a and 110 b, respectively, in accordance with the state ofthe load 150 so that the first operational amplifier 114 a operates whenthe load 150 is in the active state and the second operational amplifier114 b operates when the load 150 is in the standby state. Thus, theoperations of the two constant voltage circuits 110 a and 110 bdifferent in transient response and current consumption are switched.

When the first switching signal 140 a transmitted to the first constantvoltage circuit 110 a by the switching logic circuit 140 is at highlevel (HIGH), the n-channel MOS transistor 122 a is turned ON, and theoperational amplifier 114 a operates to control the gate voltage of theoutput transistor 116 a so that the two input voltages to theoperational amplifier 114 a are equalized. Accordingly, the outputvoltage of the first constant voltage circuit 110 a is output to theoutput terminal 130 of the constant voltage power supply.

On the other hand, when the first switching signal 140 a is at low level(LOW), the n-channel MOS transistor 122 a is turned OFF, so that thesupplying of power to the reference voltage part 112 a and the detectionresistors 118 a and 120 a is stopped. Further, the operational amplifier114 a is stopped, and the output voltage of the operational amplifier114 a is set to high level so that the output transistor 116 a is turnedOFF.

Likewise, when the second switching signal 140 b transmitted to thesecond constant voltage circuit 110 b by the switching logic circuit 140is HIGH, the output voltage of the second constant voltage circuit 110 bis output to the output terminal 130 of the constant voltage powersupply. Further, when the second switching signal 140 b is LOW, theoutput transistor 116 b is turned OFF.

The response speed of the second constant voltage circuit 110 b iscompared with that of the conventional constant voltage circuit (FIG.1). If the transistors employed in the operational amplifier 114 b andthe conventional operational amplifier 33 b have the same current supplycapacity, the operational amplifiers 114 b and 33 b are equal inresponse speed. However, the current supply capacity of the outputtransistor 116 b of the second constant voltage circuit 110 b is smallerin current by three or four digits than that of the output transistor116 a of the first constant voltage circuit 110 a. Accordingly, theoutput transistor 116 b can be extremely small in size.

Specifically, the device size ratio of the output transistor 116 a ofthe first constant voltage circuit 110 a to the output transistor 116 bof the second constant voltage circuit 110 b was set to be greater thanor equal to the drive current ratio of the operational amplifier 114 aof the first constant voltage circuit 110 a to the operational amplifier114 b of the second constant voltage circuit 110 b. In this case, thegate-source capacitance, the gate-bulk capacitance, and the gate-draincapacitance of the output transistor 116 b are extremely small comparedwith those of the output transistor 116 a. Accordingly, although thedrive capability of the operational amplifier 114 b is low, there is nosignificant reduction in response speed. As a result, the response speedof the second constant voltage circuit 110 b was dramatically improvedcompared with that of the combination of the low-speed voltagestabilization part 29 b and the output transistor 25 of the conventionalconstant voltage power supply of FIG. 1.

In the conventional constant voltage circuit 21 of FIG. 1, a large-sizeoutput transistor is required in the case of connecting two power supplycircuits in parallel, thus increasing the chip area of an IC. On theother hand, according to the embodiment of the present invention, theload current of the second constant voltage circuit 110 b is used onlyin the standby state where only approximately 1 μA to 1 mA of currentflows. Accordingly, the output transistor 116 b can be extremely smallin size. Therefore, there is no need to increase the area of the ICchip. Further, according to the embodiment of the present invention, theswitch parts 37 a and 37 b employed in the conventional constant voltagecircuit 21 of FIG. 1 are unnecessary. Accordingly, it is possible tosimplify the circuit.

FIG. 3 is a timing chart for illustrating mode switching. The switchinglogic circuit 140 outputs the first and second switching signals 140 aand 140 b at the time of mode switching so that a period of time duringwhich the first and second constant voltage circuits 110 a and 110 boperate simultaneously is provided. This period, which may be referredto as a “simultaneous ON period,” is set to be greater than the outputvoltage rising period of each of the first and second constant voltagecircuits 110 a and 110 b.

In the conventional constant voltage power supply of FIG. 1, whencurrent has been continuously supplied to the load 3 at the time of modeswitching, the driver (output transistor 25) is controlled by theoperation of the high-speed voltage stabilization part 29 a having alarge current supply capacity. Accordingly, a relatively high level ofnoise may be generated during a certain period of transition of thehigh-speed voltage stabilization part 29 a from an OFF state to a stableoperating state. On the other hand, according to the constant voltagepower supply of this embodiment, the output transistors 116 a and 116 bare simultaneously controlled by the different operational amplifiers114 a and 114 b, respectively. Therefore, either one of the outputtransistors 116 a and 116 b always operates stably. Accordingly, it ispossible to supply a load also at the time of mode switching, andtherefore, it is possible to reduce noise resulting from the operationalamplifier 33 a having a large current supply capacity. As a result, itis possible to prevent noise generated by the operational amplifier 33 aduring its transition from an OFF state to a stable operating state atthe time of mode switching.

According to the constant voltage power supply of this embodiment, thefirst constant voltage circuit 110 a that consumes a large amount ofcurrent but has excellent ripple rejection and load transient responseand the second constant voltage circuit 110 b that is inferior in ripplerejection and load transient response but consumes less current areconnected in parallel. The first constant voltage circuit 110 a iscaused to operate when the load 150 is in an active state, and thesecond constant voltage circuit 110 b is caused to operate when the load150 is in a standby state. As a result, it is possible to improvecurrent consumption by the power supply circuit when the load 150 is inthe standby state. Further, the output transistor 116 b of the secondconstant voltage circuit 110 b is reduced in size. Accordingly, there isno significant decrease in response, which can be much better thanconventionally. Moreover, since the output transistor 116 b of thesecond constant voltage circuit 110 b is reduced in size, it is possibleto prevent an increase in IC chip area.

Further, the operational amplifier 114 a of the first constant voltagecircuit 110 a employs a transistor having a greater current supplycapacity than that of the operational amplifier 114 b of the secondconstant voltage circuit 110 b. Accordingly, it is possible to reducecurrent consumption when the load 150 is in the standby state.

Further, the output transistor 116 b is smaller in device size andcurrent supply capacity than the output transistor 116 a. Accordingly,it is possible to control a decrease in response performance.

Further, the device size ratio of the output transistor 116 a to theoutput transistor 116 b is set to be greater than or equal to the drivecurrent ratio of the operational amplifier 114 a to the operationalamplifier 114 b. Accordingly, it is possible to control a decrease inresponse performance.

Further, the first and second constant voltage circuits 110 a and 110 boperate simultaneously when the state of the load 150 switches.Accordingly, it is possible to control noise when one of the first andsecond constant voltage circuits 110 a and 110 b switches to the other.

Further, the interruption circuits 122 a and 122 b that interruptthrough current are provided. Accordingly, it is possible to furtherreduce current consumption when one of the first and second constantvoltage circuits 110 a and 110 b is not selected.

Further, when the state of the load 150 switches, there is a period oftime during which both of the operational amplifiers 114 a and 114 boperate and both of the interruption circuits 122 a and 122 b are turnedon. Accordingly, it is possible to control noise when one of the firstand second constant voltage circuits 110 a and 110 b switches to theother.

The present invention is not limited to the specifically disclosedembodiment, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese Priority Patent ApplicationNo. 2003-433774, filed on Dec. 26, 2003, the entire contents of whichare hereby incorporated by reference.

1. A constant voltage power supply for supplying power to a load thatswitches between an active state and a standby state, comprising: afirst constant voltage circuit configured to apply a reference voltageto a first input terminal of a first operational amplifier, apply avoltage obtained by dividing an output voltage to a second inputterminal of the first operational amplifier, and control a first outputtransistor by an output of the first operational amplifier; a secondconstant voltage circuit configured to apply a reference voltage to afirst input terminal of a second operational amplifier, apply a voltageobtained by dividing an output voltage to a second input terminal of thesecond operational amplifier, and control a second output transistor byan output of the second operational amplifier, the second constantvoltage circuit being configured to be inferior in transient response toand consume less current than the first constant voltage circuit; and aswitching signal generation circuit configured to transmit a switchingsignal in accordance with the state of the load, wherein an input ofeach of the first and second constant voltage circuits is connected toan input terminal of the constant voltage power supply, and an output ofeach of the first and second constant voltage circuits is connected toan output terminal of the constant voltage power supply; and theswitching signal generation circuit outputs the switching signal tocause the first operational amplifier to operate when the load is in theactive state, and outputs the switching signal to cause the secondoperational amplifier to operate when the load is in the standby state.2. The constant voltage power supply as claimed in claim 1, wherein: thefirst and second operational amplifiers are equal in circuitconfiguration; and the first operational amplifier employs a transistorgreater in current supply capacity than that of the second operationalamplifier.
 3. The constant voltage power supply as claimed in claim 1,wherein the second output transistor is smaller in device size andcurrent supply capacity than the first output transistor.
 4. Theconstant voltage power supply as claimed in claim 3, wherein a devicesize ratio of the first output transistor to the second outputtransistor is greater than or equal to a drive current ratio of thefirst operational amplifier to the second operational amplifier.
 5. Theconstant voltage power supply as claimed in claim 1, wherein when theload switches from one to the other of the active and standby states,the switching signal generation circuit outputs the switching signal sothat there is a period of time during which the first and secondconstant voltage circuits operate simultaneously.
 6. The constantvoltage power supply as claimed in claim 1, wherein: each of the firstand second constant voltage circuits includes a switching circuitconfigured to be turned on to allow through current to flow and beturned off to interrupt the through current; and the switching circuitof the first constant voltage circuit is turned on and the switchingcircuit of the second constant voltage circuit is turned off when theload is in the active state, and the switching circuit of the firstconstant voltage circuit is turned off and the switching circuit of thesecond constant voltage circuit is turned on when the load is in thestandby state.
 7. The constant voltage power supply as claimed in claim6, wherein when the load switches from one to the other of the activeand standby states, the switching signal generation circuit outputs theswitching signal so that there is a period of time during which thefirst and second operational amplifiers operate and the switchingcircuits of the first and second constant voltage circuits are turnedon.